1. Technical Field
The embodiments described herein relate to a magnetic resonance imaging (MRI) apparatus and an MRI method, and more particularly, to a wideband MRI apparatus and a wideband MRI method capable of simultaneously acquiring magnetic resonance (MR) images for different kinds of nuclei.
2. Related Art
An MRI apparatus is configured to reconstruct MR images based on MR signals. A MRI apparatus typically applies a static magnetic field, a gradient magnetic field, and a radio frequency (RF) signal having a selected frequency to a subject to excite a selected nucleus type and then detect the MR signal responsively emitted by the excited nucleus. The selected frequency of the RF signal has to be tuned to be equal or close to the resonance frequency of the selected nucleus type, or the so-called Larmor frequency, which depends on the gyromagnetic ratio of the selected nucleus type and the intensity of the total magnetic field. Different kinds of nuclei have different gyromagnetic ratios and therefore have different resonance frequencies.
FIG. 1 is a table summarizing the gyromagnetic ratios for some exemplary kinds of nuclei. As shown, the gyromagnetic ratios may vary greatly between different kinds of nuclei. Accordingly, the frequency ranges to excite and detect different kinds of nuclei may differ greatly, for example, from several MHzs to several tens of MHzs.
Different kinds of nuclei reveal different information on a subject, for example, which can be related to different physiological activities or maladies. Moreover, correlative studies on MR images of different kinds of nuclei are often required to achieve more profound observation on a subject and hence more accurate disease prediction, tracking, or diagnosis, for example.
In order to acquire different information revealed by different kinds of nuclei that have significantly different excitation/detection frequency ranges, a conventional MRI apparatus employs a frequency switching strategy: it switches between different RF excitation/detection frequencies, sequentially performing imaging operations multiple times, and in each imaging operation, merely exciting and detecting a single kind of nucleus. As a result, the total imaging time is considerably lengthy.
Furthermore, the response time of the MR signals may also differ greatly among different kinds of nuclei, making the sequential MR imaging operations during a lengthy imaging course incapable of timely capturing MR images, particularly for the nuclei with very short response time. Accordingly, different MR images acquired at different times by a conventional MRI apparatus cannot faithfully demonstrate real characteristics, e.g., physiological activities, within a subject.